1. Field of the Invention
The present invention relates to inspection apparatuses for optical transmission device used in optical communication. More particularly, the present invention relates to an inspection apparatus suitable for easy inspection of a specific wavelength in optical transmission device used in a communication system employing a plurality of wavelengths.
2. Description of the Related Art
In order to effectively utilize optical fiber, simultaneous and bidirectional transmission has been made between each subscriber and a central station with a single optical fiber. FIG. 6(A) is a schematic view showing an example of a known optical communication system. This system is generally used when a plurality of optical fibers each perform bidirectional transmission, and is an optical fiber network in which optical fiber cable 103 are led from an optical transmission apparatus 101, such as an optical transmission module, in a central station 100 to subscribers 102. The optical transmission apparatus 101 is an assembly of multiple optical transmission devices 104, as shown in FIG. 6(B), and each subscriber 102 also has an optical transmission device (not shown). That is, each subscriber 102 and the central station 100 are connected by the optical fiber cable 103 through their respective optical transmission devices.
FIG. 7 shows an example of an optical transmission device that performs single-optical-fiber bidirectional transmission. In this optical transmission device 104, a light-emitting device 111 or a light-receiving device 112 is optically coupled to each fiber of an optical multi-fiber cable 103 through a PC-type or SC-type connector 105. More specifically, an optical fiber 110 connected to the connector 105, a lens 113, a wavelength selective filter 114, a lens 115, and the light-emitting device 111, such as a laser diode (LD), are arranged coaxially. The light-receiving device 112, such as a photodiode (PD), is placed perpendicular to the axis of the optical fiber 110 with a lens 116 therebetween. The wavelength selective filer 114 has a function of separating a transmission signal and a receiving signal.
The optical fiber cable 103 is led to the optical transmission device 104 and is divided into individual fibers, which are connected through the connectors 105 to the corresponding optical transmission devices 104 arranged in parallel in the horizontal direction, as shown in FIG. 6(B).
In general, an optical transmission device performs transmission and receiving using two types of light having different wavelengths, and the wavelengths used for the subscribers and the central station are different. More specifically, the optical transmission device on the subscriber side transmits light having a wavelength of 1.3 μm and receives light having a wavelength of 1.55 μm, and conversely, that of the central station transmits light having a wavelength of 1.55 μm and receives light having a wavelength of 1.3 μm, for example.
Such an optical transmission device can be used for both the subscriber and the central station by exchanging a wavelength selective filter and an LD chip. Therefore, component sharing, reduced production cost, and high economical efficiency can be achieved. Moreover, this system is extremely economical because simultaneous and bidirectional transmission can be accomplished with a single optical fiber.
In the above-related art, however, a simple visual assessment of whether the optical transmission device is provided for the subscriber or for the central station is impossible.
As described above, the optical transmission devices for the subscriber and the central station share most components except for a wavelength selective filter, an LD chip, and the like provided inside the casing. Moreover, since the single optical fiber is used, there is no difference in outer shape between the optical transmission devices. Accordingly, it is difficult to discriminate between the optical transmission devices by appearances. Conventionally, the optical transmission devices can be discriminated only by the type identifier printed on the surface of the casing.
Therefore, if an optical transmission device for a subscriber and an optical transmission device for a central station are inadvertently interchanged, that is, for example, if an optical transmission device for a subscriber is installed in the central station, it is difficult to check which optical transmission device is placed in the wrong position. In this case, it is time consuming to construct the system, work efficiency is reduced, and the cost is increased.
The above-described drawback has promoted a demand to achieve an inspection apparatus that can easily discriminate between an optical transmission device for a subscriber and an optical transmission device for a central station when laying a communication system that is capable of simultaneous and bidirectional transmission with a single optical fiber.